\documentclass{article}


%%-----------PGF/TIKZ
\usepackage{pgf}
\usepackage{tikz}
\usetikzlibrary{arrows,shapes,shapes.arrows,positioning}

\usepackage{palatino}

% Use preview.sty to crop the page
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{pgfpicture}
\setlength\PreviewBorder{0pt}

\begin{document}

\begin{figure}
\begin{tikzpicture}
    [
    node distance=10mm and 10mm,
    scale=.8,
    block/.style ={
        rectangle,
        draw=black, 
        thick, 
        fill=gray!20, 
        text width=5.5em,
	    text centered,
    },
    ]

    \node(rtext) [text width=7em,text centered] {Precipitation, Inflow};
    \node(wpge1) [block,right=of rtext] {Web Page};
    \node(ttxt1) [text width=12em,text centered,above=of wpge1] {Optional User Inputs};
    \node(btext) [text width=12em,text centered,below=of wpge1] {Previous Results};
    \node(model) [block,right=of wpge1] {Simulation Model};
    \node(wpge2) [block,right=of model] {Web Page};
    \node(ttxt2) [text width=12em,text centered,above=of wpge2] {Output to User};

    \path[draw,thick,-stealth'] (rtext) -- (wpge1);
    \path[draw,thick,-stealth'] (ttxt1) -- (wpge1);
    \path[draw,thick,-stealth'] (btext) -- (wpge1);
    \path[draw,thick,-stealth'] (wpge1) -- (model);
    \path[draw,thick,-stealth'] (model) -- (wpge2);
    \path[draw,thick,-stealth'] (wpge2) -- (ttxt2);

\end{tikzpicture}
\end{figure}
\end{document}
